Audio signals are typically transmitted as subcarriers modulated onto the main video carrier, a form of frequency division multiplexing. For example, NTSC subcarriers are inserted in the 5.0 to 8.5 MHz range. PAL and SECAM subcarriers often have 6.60 and 7.20 MHz center frequencies but, as is the case with NTSC transmissions, other frequencies are frequently selected. These audio subcarriers may be used to transmit monophonic or stereo sound. In North America one of four systems is typically used to transmit stereo sound over satellite circuits: multiplex, Warner Amex, discrete and processed narrow deviation stereo. The Wegener Panda stereo system is commonly used on European satellites.